This application has three main goals. In the first goal, Dr. Balch's group proposes to extend their high-resolution structural analysis of GDIs. They propose to solve several structures that include the structure of the related beta GDI, the structure of Rab3A in the GDP form, to determine the structure of the geranyl lipid binding site in alpha GDI and to solve the structure of a complex made of alpha GDI and Rab3A. This part of the project is a continuation of a fruitful collaboration with the group of Dr. I. Wilson at Scripps. The second goal is to identify the membrane protein(s) involved in the recruitment to and retrieval from GDI to membranes that are required during the process of vesicle formation and fusion. These studies will be based on biochemical approaches and are built on the hypothesis that domain II of GDI uses a mobile loop as the effector domain for these interactions. The third goal is to study the role of GSI, Rab3A and other possible interacting proteins in the process of vesicle delivery during exocytosis. Dr. Balch will use an in vitro, semi-intact synaptosome assay recently developed by his group to explore the functional role of these proteins.